Why am I still spotting? ):

Update to my last post... my period ended the 25th of June. Then 4 days later, I had a little bit of blood left, it was old brown blood. Ever since then, I've been having brown discharge and it won't go away. Now today, I went to the bathroom to find more fresh blood, but not a lot. It has turned into brown blood again. The details I can provide are that I'm not on birth control, I've recently been exercising at the gym for a couple of weeks now (weight lifting and intense cardio) I'm not experiencing any other side effects besides this brown discharge and random frsesh blood, and I AM sexually active. I last had PROTECTED sex 3 days ago. (We were super safe and responsible) My friends are worried and think that I should go to a doctor because they say that this is abnormal. What's going on? Should I be worried that my body has been doing this for about a week?